The number of complaints against airlines from disabled travelers doubled in a decade to more than 30,000 in a year, according to a federal watchdog report Wednesday.

But 12 carriers studied each demonstrated they have repeated training programs for helping passengers with disabilities, according to the Government Accountability Office.

Airlines provide a combination of classroom and computer-based training. The training typically includes an eight-hour class with lessons in handling wheelchairs and assisting with boarding, according to the 34-page GAO report required by a law last year governing the Federal Aviation Administration.

“Airlines place a high priority on meeting the needs of all passengers and regularly invest in training programs and processes, new and improved technologies and customer support staff to accommodate passengers with disabilities,” said Kathy Grannis Allen, spokeswoman for Airlines for America, a trade group representing most of the largest carriers. “A4A and our members will continue to work collaboratively with all parties to ensure that our customers are provided with any support necessary and the highest levels of customer service.”

The Transportation Department investigates complaints and can impose fines. The department has been studying with airlines and advocacy groups for ways to smooth travel for people with disabilities.

Since 2005, the department investigated 51 cases involving disabilities, according to GAO. Enforcement actions included warning letters or consent orders for corrective action.

Airlines faced $1.675 million in fines in 13 of those cases, paying $887,500 to the government and either investing in corrective actions or avoided the remaining $787,500, according to GAO. The corrective measures included compensation to passengers, better wheelchair tracking and training for workers.

From 2005 through 2015, GAO found the number of disability complaints filed with airlines grew from 13,584 per year to 30,289. The number of disability complaints filed directly with the department also rose from 511 to 944 during that period.

Those increases occurred as the number of passengers grew from 746 million to 806 million.

More than half the complaints in 2015 – 17,828 to airlines and 463 to the department – dealt with airlines failing to provide assistance, which mostly meant a lack of help with wheelchairs, according to GAO.

The next largest categories of complaints to airlines involved seating accommodation (3,026), a problem with a service animal (1,638), storage or delay of an assistive device (1,570) and damage to a device (1,541).